I’ve been taking a look at Clojure lately, as a JVM friendly flavour of Lisp I’ve got to say it looks pretty interesting. One problem that I’ve had though is that all of the documentation out there (of which there’s very little, to be honest) seems to assume that you’ll be writing/running a pure Clojure program as you main application. There’s plenty of information about calling Java code from Clojure programs, and some information about extending Java classes and interfaces with Clojure code, but nothing about getting the two talking together at runtime. So here’s how it’s done.

First off you need to set up the symbols and namespaces that you are going to need to start up a clojure environment.

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Symbol main = Symbol.create("main");
Symbol clojureMain = Symbol.create("clojure.main");
Symbol user = Symbol.create("user");
Symbol require = Symbol.create("require");
Namespace userNS = Namespace.findOrCreate(user);
Namespace clojureMainNS = Namespace.findOrCreate(clojureMain);

Once you have these you can require the clojure main namespace, this is the same one that is used to run scripts or start a REPL from the comand line.

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Var.intern(RT.CLOJURE_NS, require).invoke(clojureMain);

Then, and this is the bit that I had trouble working out, you need to bind your application’s domain model (or at least those bits of it that you want to expose) into the user namespace in Clojure.

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for (Map.Entry<String, Object> global : globals.entrySet()) {
    String key = global.getKey();
    Object value = global.getValue();
    Var.intern(userNS, Symbol.create(key), value);
}

Finally you’re ready to grab the main method and run it.

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Var.intern(clojureMainNS, main).applyTo(RT.seq(new String[0]));

The emtpy string array here is emulating an emty sargument list at the command line. Some things to note here: you need to intern vars before you can use them, even for core library features like require, this surprised me at first but when you remember that most Lisps are built around a very small core of special forms with everything else defined in Lisp, it makes some sense.